Objective The swordtail fish (Xiphophorus helleri) was employed to clone the gene sequence of immunoglobulin D (IgD), and the tissue distribution and expression laws after vaccine immunization of the IgD in swordtail fish were explored.

Method With the known EST libraries, the cDNA sequences of IgD gene from swordtail fish were cloned and analyzed, and the tissue distribution and the relative expression changes in head kidney, spleen and intestine after vaccination were detected by real-time PCR.

Result The complete open reading frame (ORF) of swordtail fish was 3 879 bp, which encoded 1 298 amino acids, predicting that the molecular weight was 145.5 ku. The IgD of swordtail fish was composed of variable region and constant region, and the constant region contained CH1-CH2-CH3-CH4-CH5-CH6-CH7 and a transmembrane region (TM). The systematic evolution analysis of gene sequences showed that swordtail fish IgD shared similarities of 70% with mandarin fish (Siniperca chuatsi). IgD mRNA expression of swordtail fish was highly detected in head kidney, spleen and intestine. After being immunized with Aeromonas hydrophila vaccine, the expression of IgD in the head kidney and spleen was significantly down-regulated, and it was significantly up-regulated in the intestine.

Conclusion IgD may play an important role in immune response and intestinal mucosal immunity of swordtail fish.